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Full text of "Directions for industrial use of aerosols"

BOARD 

October 1953 LIBRARY E-835, Revised w 

STATE PLANT BOARD 

United States Department of Agriculture 

Agricultural Research Administration 

Bureau of Entomology and Plant Quarantine 



DIRECTIONS FOR INDUSTRIAL USE OF AEROSOLS 

By A. H. Yeomans 
Division of Stored Product Insect Investigations 



Aerosols are used in many industrial structures to control insects 
by direct contact or by applying a light deposit of insecticide on the top 
of exposed horizontal surfaces. 

An aerosol is composed of a number of fine liquid particles suspended 
in the air. It is considered that in an insecticidal aerosol all the particles 
should be smaller than 50 microns in diameter and 80 percent (by weight) 
of them should be less than 30 microns. Vapors or smokes composed 
of particles less than 0.1 micron in diameter are not classed as insecti- 
cidal aerosols because they exhibit slightly different characteristics. 

Aerosols may be produced by liquefied-gas formulations released 
through capillary or expansion-chamber nozzles, by steam or air 
atomization of liquid, by spinning disks and rotors, by forcing liquid 
under high pressure through atomizing nozzles, by heat vaporization, 
or by a combination of these methods. 

A wide range of particle sizes can be produced, and the size of the 
particles has a great influence on the effectiveness of the aerosol. The 
particle size determines the time the aerosol remains suspended in the 
air and therefore the amount of dispersion by air currents throughout 
the enclosure. The particle size is a critical factor influencing the 
amount that collects on an insect as it flies through the aerosol. If the 
particles are too small, they are deflected from the flying insect as 
smoke is from a moving automobile, but they will settle out on the insect 
while it is at rest if the exposure time is adequate. If they are too large, 
they settle rapidly and their dispersion is poor; therefore their chance 
of contacting the insect is also poor. When an insect does collide with 
an oversized droplet, the excess insecticide is wasted. 



- 2 



Settling Rate and Dispersion 

Aerosol particles tend to settle vertically at a rate related to their 
size. The time required for oil particles to settle 10 feet is given 
below. 



Diameter 
in microns 



Time 



Diameter 
in microns 



Time 



1 26 1/2 hours 

5 72 minutes 

10 19 

15 15 



20 5 minutes 

30 2 

50 45 seconds 

100 11 



Water droplets settle slightly faster. The lateral dispersion of 
aerosols is accomplished by air currents after the small impetus from 
the atomizer is expended. Aerosol particles will not be conveyed into 
dead-end cracks or into materials through which air does not circulate. 

As would be expected, small particles disperse laterally to greater 
distances than do larger ones, as they are suspended in air for a longer 
time. In unheated buildings air currents are at a minimum, but heating 
sets up convection currents that are a great aid to dispersion. In some 
cases it is necessary to aid dispersion with large-volume air blowers. 
Table 1 shows the dispersion of aerosols of various particle sizes in an 
unheated room with a ceiling height of 8 feet. 

Table 1. --Lateral dispersion of aerosols of various particle 
sizes released at the ceiling level in an unheated room 
8 feet high 



Feet from 
source 



Percent of particles of indicated 
mass median diameter 



5 microns 



15 microns 



25 microns 



45 microns 








28 


43 


66 


84 


10 




25 


40 


29 


15 


20 




20 


13 


4 


Trace 


30 




10 


4 


Trace 




40 




7 


Trace 






50 




5 








75 




4 








100 


T 


race 









-3 



Deposit 

About 95 percent of an aerosol settles on the top of horizontal sur- 
faces, and the remaining 5 percent on the walls and ceilings. The 
amount of deposit on a horizontal surface depends on the concentration 
of the aerosol directly above that surface. Therefore, if the aerosol is 
evenly dispersed throughout a room, the deposit will be proportional to 
the distance of the surface from the ceiling. The deposit on walls and 
ceiling is mainly on small protrusions, such as fibers, on rough sur- 
faces which catch the particles as they go by on air currents or by 
settling. Insects resting on the walls and ceiling will also be struck 
by an occasional particle as it passes by. Surfaces colder than the air 
temperature slightly attract very small particles. 

Selecting Particle Size 

No single particle-size range is suitable for all conditions where an 
aerosol may be used. The particle-size range should be selected after 
the factors involved in a proposed treatment have been evaluated. The 
most important factor is time. 

If the aerosol is to be applied to a structure or room that can be 
closed for several hours, small particles will give the best dispersion 
and penetration into small crevices. An aerosol with particles of about 
5 microns mass median diameter was applied in a large warehouse. 
A thermal generator was operated outside the building, introducing the 
aerosol through an open door. The aerosol dispersed along the ceiling, 
and by the time the proper amount had been applied it was well distrib- 
uted throughout the warehouse by convection currents. The warehouse 
was tfren closed overnight and the aerosol allowed to settle. Uniform 
distribution resulted. 

When the treatment must be limited to a short time, larger particles 
are necessary. A 15-minute exposure is sufficient with an aerosol having 
particles with a mass median diameter of 15 microns, as most of the 
particles will settle 10 feet within 5 to 15 minutes. Where flying insects 
are to be controlled, this type of treatment is quite satisfactory. How- 
ever, since lateral dispersion is restricted when particles of this size 
are used, in large rooms or structures the aerosol must be released 
from several points to give uniform dispersion. 

Many types of application will fall between these two extremes. The 
proper relation between time of exposure and particle size can be cal- 
culated from the tabulation on page 2. The allowable settling time should 
be based on the smallest particles. The approximate distances for 
uniform dispersion can be determined from table 1. Better dispersion 
can be obtained when heating sets up convection currents or when fans 
are used. 



UNIVERSITY OF FLORIDA 



3 1262 09239 6349 

After considering the foregoing factors, the operator can select the 
most desirable particle size to meet his need, and then the type of gen- 
erator that will produce the desired particle size and volume of aerosol. 
If the available equipment will not produce the desired particle size, 
then the time will have to be adjusted to the size that can be produced. 

It has been demonstrated that particles above certain sizes, when 
composed of some solvents and insecticides used in aerosol formula- 
tions, will injure plant foliage. Persons treating greenhouses with 
aerosols should keep this factor in mind when selecting the particle size. 

Some structures may be too open for the successful use of aerosols 
because of too much loss through the wall openings. In large open ware- 
houses full of tobacco hogsheads, where an aerosol could not be con- 
tained in the structures, it was found that a spray with particles of 
abcut 50 microns mass median diameter, blown over the top of the 
hogsheads by a mist blower, gave a uniform deposit of insecticide and 
good insect control. 

Formulations and Dosages 

We have used the following formulations and dosages indoors: 

1 pound of technical DDT dissolved in 7 1/2 pints of Sovacid 544C 
(Socony Vacuum) to make 1 gallon, applied at the rate of 1 gallon 
per 100,000 cubic feet. 

1 1/4 pounds of technical DDT and l/4 pound of lindane dissolved in 
3 quarts of tetrachloroethylene plus 1 pint of SAE 50 motor oil 
to make 1 gallon, applied at the rate of 2 quarts per 100,000 
cubic feet. 

1 quart of synergized pyrethrum (1-10 mixture of pyrethrins and 
a synergist) plus 3 quarts of deodorized kerosene and 4 quarts 
of tetrachloroethylene (proportions of last two materials can 
be varied to regulate the particle size), applied at the rate of 
1/2 to 3 gallons per 100,000 cubic feet, depending on the insects 
involved. 

To prevent explosion hazard, aerosols containing oils should not be 
applied at rates that will give more than 2 gallons of these solvents per 
100,000 cubic feet, and they should not be released near an open flame. 
All work indoors should be done while wearing a proper respirator. The 
pyrethrum formula is recommended for use around exposed foodstuffs. 

Some formulations contain enough of a relatively nonvolatile oil to 
maintain the desired particle size while it is suspended in the air.